1. Field of the Invention
The present invention relates to a steering system for use in a motor vehicle having steerable front and rear road wheels.
2. Description of the Relevant Art
Applicant has previously disclosed a steering system for use in a motor vehicle having steerable front and rear road wheels in Japanese Patent Application No. 62-189705. In the disclosed steering system, a front wheel steered angle .delta.f and a vehicle speed V are detected, and a rear wheel objective steered angle .delta.ro is determined from the detected steered angle .delta.f and the vehicle speed V. Then, the rear wheels are turned through the rear wheel objective steered angle .delta.ro. When the motor vehicle with steerable front and rear road wheels makes a turning motion at a high speed, the rear road wheels are turned in the same direction as the front road wheels to increase a rear wheel steered angle .delta.r, thus increasing a slip angle thereof. Therefore, a cornering force produced by the tires of the rear road wheels is increased for increased vehicle stability.
With the proposed steering system, in order to give the driver a better steering feeling, a change in a steering variable such as a steering speed Ns or the like is detected to predict the intention of the driver, and the rear wheel steered angle .delta.r is adjusted according to the rate of change of the steering variable. Consequently, a deviation that the driver may feel in the steering operation can be compensated for.
In the motor vehicle of the above type, both front and rear road wheels are turned to cause the tires thereof to produce cornering forces. Therefore, a lateral acceleration (lateral G) and a yawing moment are generated immediately in response to the turning of the steering wheel. The turning behavior of the motor vehicle is governed by the lateral acceleration and the yawing moment, and so is the steering feeling which the driver has. Particularly, the steering feeling is strongly affected by a time delay (hereinafter referred to as a lateral G phase delay) occurring after the steering wheel is turned and until the lateral G is generated. If the lateral G phase delay is large, the response of the motor vehicle to the steering action initiated by the driver becomes poor, failing to give the driver a desirable steering feeling. In order to achieve a better steering feeling, therefore, it is desirable that the lateral G phase delay be adjusted dependent on the vehicle speed, particularly be reduced in a high vehicle speed range.
The present invention has been made in view of the aforesaid desideraturn in the previously disclosed steering system for use in a motor vehicle having steerable front and rear road wheels.